Re: [SPAM] [PATCH] 9pfs: fix crash in v9fs_walk()

[Greg Kurz]

On Wed, 1 Sep 2021 18:15:10 +0200
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> v9fs_walk() utilizes the v9fs_co_run_in_worker({...}) macro to run the
> supplied fs driver code block on a background worker thread.
> 
> When either the 'Twalk' client request was interrupted or if the client
> requested fid for that 'Twalk' request caused a stat error then that
> fs driver code block was left by 'break' keyword, with the intention to
> return from worker thread back to main thread as well:
> 
>     v9fs_co_run_in_worker({
>         if (v9fs_request_cancelled(pdu)) {
>             err = -EINTR;
>             break;
>         }
>         err = s->ops->lstat(&s->ctx, &dpath, &fidst);
>         if (err < 0) {
>             err = -errno;
>             break;
>         }
>         ...
>     });
> 
> However that 'break;' statement also skipped the v9fs_co_run_in_worker()
> macro's final and mandatory
> 
>     /* re-enter back to qemu thread */
>     qemu_coroutine_yield();
> 
> call and thus caused the rest of v9fs_walk() to be continued being
> executed on the worker thread instead of main thread, eventually
> leading to a crash in the transport virtio transport driver.
> 
> To fix this issue and to prevent the same error from happening again by
> other users of v9fs_co_run_in_worker() in future, auto wrap the supplied
> code block into its own
> 
>     do { } while (0);
> 
> loop inside the 'v9fs_co_run_in_worker' macro definition.
> 
> Full discussion and backtrace:
> https://lists.gnu.org/archive/html/qemu-devel/2021-08/msg05209.html
> https://lists.gnu.org/archive/html/qemu-devel/2021-09/msg00174.html
> 
> Fixes: 8d6cb100731c4d28535adbf2a3c2d1f29be3fef4
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> Cc: qemu-stable@nongnu.org
> ---

Reviewed-by: Greg Kurz <groug@kaod.org>

>  hw/9pfs/coth.h | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/9pfs/coth.h b/hw/9pfs/coth.h
> index c51289903d..f83c7dda7b 100644
> --- a/hw/9pfs/coth.h
> +++ b/hw/9pfs/coth.h
> @@ -51,7 +51,9 @@
>           */                                                             \
>          qemu_coroutine_yield();                                         \
>          qemu_bh_delete(co_bh);                                          \
> -        code_block;                                                     \
> +        do {                                                            \
> +            code_block;                                                 \
> +        } while (0);                                                    \
>          /* re-enter back to qemu thread */                              \
>          qemu_coroutine_yield();                                         \
>      } while (0)